The present invention relates to a light emitting semiconductor device. More particularly, the present invention relates to a high efficient semiconductor laser having a stable operation at a high speed and being operable by a low threshold current for laser oscillation.
Upon applying a double heterojunction structure designed by I. Hayashi and M. B. Panish in 1970 to an injection type semiconductor laser invented in 1962, the semiconductor laser has been greatly advanced in practical use. For example, the semiconductor laser becomes capable of continuous wave oscillation at room temperature as referred to "J. APPL. PHYS.", I. Hayashi, M. B. Panish, 41, 150 (1970).
FIG. 1 is a structural diagram showing its sectional view of the double heterojunction (DH) semiconductor laser of the injection type.
In FIG. 1, a first cladding layer 22 of n-type Al.sub.x Ga.sub.1-x As, an active layer 23 of undoped GaAs, a second cladding layer 24 of p-type Al.sub.x Ga.sub.1-x As and p-type GaAs layer 25 are formed in the above stated order on a substrate 21 of n-type GaAs. Further, the semiconductor laser device thus constructed is subjected to formation of an ohmic electrode 27 on the back surface of the substrate 21 and an ohmic electrode 26 of stripe type on the upper surface of the p-type GaAs layer 25. In particular, a refractive index of the active layer 23 is greater than those of the cladding layers 22 and 24 which sandwich the active layer 23 therebetween. The three layers thus constructed constitutes a refractive index-waveguiding structure.
In the double heterojunction structure, electrons and holes are injected respectively from the n-type (first) cladding layer 22 and the p-type (second) cladding layer 24 having wide energy band-gaps into the active layer 22 having a narrow energy band gap so as to confine them within the active layer. That is a so-called "carrier confinement effect". Accordingly, if the active layer 23 is formed of a thin layer, a high injection carrier density, that is, a high gain is obtained by using a small amount of current. Furthermore, with the refractive index-waveguiding structure, light caused by recombination propagates only within the active layer 23 having a high refractive index so that the scattering of the light is prevented (optical confinement) and the threshold current for oscillation is lowered.
Oscillation spectrum of the double heterojunction laser device thus constructed is made to a single mode oscillation in DC modulation by dispensing a suitable carrier confinement in the transverse direction thereto.
When the double heterojunction laser device is used in high-speed modulation, however, a multi mode oscillating spectra consisting of plural axial mode oscillating spectra are observed as shown in FIG. 2a. In order to prevent the multi mode of oscillating spectra in high-speed modulation, there have been proposed, DFB (Distributed Feedback) laser described in "Applied Physics Letter", J. Kuroda, 33, 173 (1978) and DBR (Distributed Bragg Reflector) laser described in "Electronics Letter", Y. Abe et al., 18, 410 (1982). FIG. 3a is a structural diagram showing the sectional view of the DFB laser device. FIG. 3b is a structural diagram showing the sectional view of the DBR laser device. Particularly, in the DBR laser device shown in FIG. 3b, an active layer 43 of undoped GaAs is surrounded by wave guiding-layers 41 and 42 of undoped AlGaAs.
In these laser devices, diffraction gratings 40 are formed in the active layer or layers in the vicinity of the active layer as shown in FIGS. 3a and 3b so that a single longitudinal axial mode oscillation is steadily realized. The DBR laser device produces a single oscillating spectrum in high-speed modulation as shown in FIG. 2b from which it is clear that a multi mode oscillation such as that of FIG. 2a is not occurred.
On the other hand, the DFB laser device shown in FIG. 3a has the diffraction grating structure 40 in the vicinity of the active layer 43 so that a favorable crystallinity in the active layer 43 can not be obtained. Also, since the active region and the diffraction grating region can not be independently provided, it is difficult to operate laser oscillation at low threshold current. Further, there occurs a chirping phenomenon in high-speed modulation. Therefore, the DBR laser has been more hopeful in practical use rather than the DFB laser.
Since laser light is largely attenuated in layers having the same composition as that of the active layer even in the DBR laser, however, the wave guiding layers 41 and 42 under or above the diffraction grating configuration 40 must have a different composition from that of the active layer 43 as shown in FIG. 3b. Accordingly, there has been a need for complicated and difficult processes for growth of these layers in the DBR laser device.